1. Field of the Invention
The present invention relates to a color cathode ray tube and, more particularly, to a color cathode ray tube having a shadow mask, and a method of manufacturing the same.
2. Description of the Related Art
Generally, a shadow mask type color cathode ray tube has a glass envelope constituted by a substantially rectangular faceplate, a skirt portion continuous to the faceplate, a cylindrical neck opposing the faceplate, and a funnel connecting the skirt portion and the neck. A phosphor screen on which phosphors that emit light in red, blue, and green are regularly arranged is formed on the inner surface of the faceplate. An electron gun for emitting a plurality of electron beams corresponding to red, blue, and green is disposed in the neck.
A shadow mask having a large number of regularly arranged electron beam apertures is disposed at a position closely opposing the phosphor screen at a predetermined distance. The peripheral portion of the shadow mask is bonded to a mask frame and is engaged with stud pins of the skirt portion through a mask holder. Each electron beam aperture of the shadow mask is formed such that the sectional area of an opening on the phosphor screen side (to be referred to as a larger opening hereinafter) is larger than that of an opening on the electron gun side (to be referred to as a small opening hereinafter). With this shape, a constant electron beam amount is maintained even when an electron beam is obliquely incident on the electron beam aperture at the peripheral portion of the shadow mask.
In the color cathode ray tube having the above arrangement, the shadow mask has a function of transmitting the electron beam therethrough such that the electron beam correctly lands on only the phosphor of each color which is geometrically in a one to one relationship with the electron beam aperture. This is a significant element called a color selection electrode. The electron beam apertures of the shadow mask may be circular or rectangular in shape. Usually, shadow masks having circular apertures are used in display tubes that display characters and figures at high definition. Shadow masks having rectangular apertures are mainly used in tubes used in the household, such as television tubes. For example, a rectangular electron beam aperture is formed such that its longer side extends substantially perpendicular to the shorter side (vertical axis) of a substantially rectangular faceplate. A large number of vertical aperture columns each having a plurality of vertically arranged apertures are arranged in the horizontal direction. The adjacent shorter sides of the electron beam apertures of the respective vertical aperture arrays are arranged with bridge portions therebetween, which extend substantially in parallel to the longer side (horizontal axis) of the faceplate.
The closer to the peripheral portion of the shadow mask, the larger the angle of incidence of the electron beam, i.e., the larger the angle defined by the normal to the mask or the aperture central axis and the electron beam axis, and part of the incident electron beam collides against the aperture edge or aperture wall of the aperture at a higher rate. As a result, the shape of the electron beam spot formed on the phosphor screen is distorted, thereby degrading the luminance or white uniformity.
In recent years, an image which reflects less external light and has less distortion is demanded from the viewpoint of human technology. A flat panel is inevitable to satisfy this demand. Accordingly, a flat shadow mask having a relative relationship with the phosphor screen is required. In a flattened shadow mask, the angle of incidence of the electron beam becomes inevitably large, and in particular, the angle of beam incidence at the peripheral portion of the mask increases. As a result, a distortion in the beam spot shape also becomes conspicuous.
The problem of beam spot distortion is more likely in a shadow mask made of a thick material and in a shadow mask having electron beam apertures which are arranged at small pitches so as to obtain a high resolution.
As means for preventing the beam spot distortion or beam omissions, Jpn. Pat. Appln. KOKOKU Publication No. 47-7670 and Jpn. Pat. Appln. KOKAI Publication Nos. 50-142160 and 57-57449 propose a so-called off-center mask in which the aperture center of the phosphor-screen-side larger opening of the shadow mask is deviated with respect to the aperture center of its corresponding electron-gun-side smaller opening in a direction in which the electron beam passes. With the arrangement of this off-center mask, the problem in which the incident electron beam collides against the aperture wall surface or aperture edge of the larger opening to cause a beam omission can be avoided.
However, in the off-center mask, the amount of electron beam passing through the electron beam aperture, i.e., the width of the passing electron beam is determined by the position of that portion of the end edge of the smaller opening which is located at the mask center side, and the position of that portion of the boundary between the larger and smaller openings which is located outward in the radial direction with respect to the mask center. In this case, part of the electron beam incident on the electron beam aperture is shielded by that portion of the wall surface defining the smaller opening which is located outward in the radial direction with respect to the mask center, and the width of the actual passing electron beam becomes smaller than the diameter of the smaller opening. The difference between the width of the passing electron beam and the aperture diameter of the smaller opening is increased in a flat square tube. When the position of the boundary, i.e., the distance from the end edge of the smaller opening on the shadow mask surface to the boundary is changed, the width of the passing electron beam is also changed. This causes a degradation in white uniformity in a color cathode ray tube in which the electron beam landing area on the phosphor screen has little freedom.
Furthermore, in a flat shadow mask, an electron beam collides against that portion of the wall surface defining the smaller opening which is located outward in the radial direction with respect to the mask center, and is reflected by this portion at a higher rate. This is caused by the following facts. Usually, the electron beam apertures of a shadow mask are formed by etching. Thus, the angle defined by the aperture center axis of the smaller opening and that portion of the side surface of the smaller opening which is located near the opening edge on the electron-gun-side becomes smaller than the angle defined by the aperture center axis of the smaller opening and that portion of the side surface of the smaller opening which is located near the boundary. When an offset between the smaller and larger openings is large, the boundary on the electron beam traveling side approaches the electron gun side, and the angle defined by the side surface of the smaller opening, against which the electron beam collides, and the aperture central axis is decreased. As a result, the reflected electron beam directed to the center of the phosphor screen is increased. Since this reflected electron beam is not controlled at all, it lands on a phosphor other than the predetermined phosphor to cause it to emit light, so that the black level of the entire screen is decreased, thereby largely decreasing the contrast. As a result, the contrast becomes the same as that obtained when the TV screen is observed during daylight, and the image quality is degraded.
Even when the aperture centers of the larger and smaller openings are deviated from each other by a necessary amount so that the electron beam will not collide against the aperture side surface of the electron beam aperture or the large opening end to cause beam spot distortion in this manner, occurrence of an undesired reflected electron beam, that causes degradation in contrast, cannot be avoided in a color cathode ray tube having a flattened shadow mask and a large angle of incidence of the electron beam.